Add Dev Container Feature installation scenarios

[ocularminds]

Context

The Dev Container Feature validates metadata and shell syntax, but it lacks end-to-end installation scenarios for published and local CLI artifacts.

Scope

Exercise the Feature in representative base images using both a pinned CLI version and the local tarball override.

Acceptance criteria

• Tests install successfully on at least Debian/Ubuntu and Alpine-compatible scenarios, or unsupported bases are explicitly documented.
• Both version=latest and an explicit version are covered.
• DECIONIS_CLI_TARBALL is tested without network access.
• A checksum mismatch fails without installing a binary.
• The installed decionis version matches the requested artifact.